With disease or damage, the normally smooth, lubricious cartilage covering joint surfaces progressively deteriorates, exposing bone and leading to arthritic pain that is exacerbated by activity and relieved by rest. Today, patients with osteoarthritis are faced with only one of two choices: either manage their pain medically, or undergo an effective but highly bone-sacrificing surgery. Medical management includes weight loss, physical therapy, and the use of analgesics and nonsteroidal anti-inflammatories. These can be effective at reducing pain but are not curative. Other options include drugs like glucosamine or hyaluronan to replace the “lost” components of cartilage, but despite their extensive use in the U.S., their efficacy is still questioned. When medical intervention fails and a patient's joint pain becomes unbearable, surgery is advised. Total joint arthroplasty is a surgical procedure in which the diseased parts of a joint are removed and replaced with new, artificial parts (collectively called the prosthesis). In this highly effective but invasive procedure, the affected articular cartilage and underlying subchondral bone are removed from the damaged joint. A variety of replacement systems have been developed, typically comprised of ultra-high molecular weight polyethylene (UHMWPE) and/or metals (e.g. titanium or cobalt chrome), or more recently, ceramics. Some are screwed into place; others are either cemented or treated in such a way that promotes bone ingrowth. These materials have been used successfully in total joint replacements, providing marked pain relief and functional improvement in patients with severe hip or knee osteoarthritis.
A large number of patients undergo total hip arthroplasty (THA) in the US each year, which involves implanting an artificial cup in the acetabulum and a ball and stem on the femoral side. The goals of THA are to increase mobility, improve hip joint function, and relieve pain. Typically, a hip prosthesis lasts for at least 10-15 years before needing to be replaced. Yet despite its success as a surgical procedure, THA is still considered a treatment of last resort because it highly “bone-sacrificing,” requiring excision of the entire femoral head. It is this major alteration of the femur that often makes revision replacement difficult. While this procedure has a survival rate of 90% or more in the elderly (who usually do not outlive the implant), implant lifetimes are significantly shorter in younger, more active patients. As a result, younger patients face the prospect of multiple, difficult revisions in their lifetime. Revisions are required when implants exhibit excessive wear and periprosthetic bone resorption due to wear particles, as well as aseptic loosening of the prosthesis resulting from stress shielding-induced bone resorption around the implant.
The aforementioned limitations of THA have prompted the industry to seek less bone-sacrificing options for younger patients, with the hope that a THA can be postponed by at least five years or more. One approach towards improving treatment has been to develop less invasive surgical procedures such as arthroscopic joint irrigation, debridement, abrasion, and synovectomy. However, the relative advantage of these surgical techniques in treating osteoarthritis is still controversial. An alternative to THA is hip “resurfacing,” has now re-emerged because of new bearing surfaces (metal-on-metal, rather than metal-on-polyethylene). While many patients can expect to outlive the procedure's effectiveness, hip resurfacing preserves enough bone stock on the femoral side to allow for later total hip replacement. Unfortunately, there are enough potential drawbacks that doctors offering hip resurfacing say that the procedure should still be deferred as long as possible. In metal-on-metal resurfacing, the femoral head is shaped appropriately and then covered with a metal cap that is anchored by a long peg through the femoral neck. It requires a more precise fit between the cap and cup, and the procedure generally sacrifices more bone from the acetabulum compared to conventional replacements due to the larger diameter of the femoral component. Furthermore, a resurfacing operation has a steep learning curve and takes longer than a THA. Femoral neck fractures caused by bone resorption around the peg have been reported, and the long-term impact of metal ion release from the bearing surfaces is also not yet known in humans. As a result of these complications, today's resurfacing devices are still only indicated in patients for whom hip pain is unbearable, as is the case for THA.
The present invention addresses the needs in the art and provides an interpenetrating polymer network hydrogel that is strain-hardened by means of swelling that forms the basis of an arthroplasty device and a method for making this device.